Several years ago, we noted during electrophysiological studies that a number of neurons in the lumbar spinal cords of rats could be antidromically activated from the hypothalamus using small amounts of current. These findings suggested the existence of a projection that carries somatosensory and nociceptive information from the spinal cord directly to the hypothalamus. We used several anatomical and physiological techniques to confirm the existence of this projection. Interestingly, our retrograde tracing studies indicated that in rats the total numbers of spinohypothalamic tract and spinothalamic tract neurons are similar. In thoracic and sacral levels of the cord, there are considerably more spinohypothalamic tract neurons than spinothalamic tract neurons. The spinohypothalmic tract is of interest because it may be an important source of input to areas of the hypothalamus that are thought to play important roles in autonomic, neuroendocrine and affective responses to somatosensory stimuli. In the first series of studies, we will examine the response characteristics of SHT neurons in monkeys. In the second, we will determine the location of spinothalamic tract axons in the spinal cord white matter in monkeys. In the third set of studies, we will determine the functional characteristics of the large number of spinohypothalamic tract neurons in the thoracic spinal cord of rats. In these studies, we will use quantitative stimulation of cutaneous and visceral structures. We will also test the hypothesis that SHT neurons are capable of providing sensory information originating in the nipples to hypothalamic neurons involved in the milk-ejection reflex. In the fourth series of studies, we will physiologically characterize SHT neurons in segments C1-2 of rats. We will also record from such neurons intracellularly and inject them with neurobiotin. These experiments should reveal the complete morphology of SHT cell bodies and dendrites. In addition, they should provide detailed information on the axons of such neurons and their collateral projections within the brainstem. We will also record from SHT units intraaxonally in the posterior diencephalon and inject them with neurobiotin. These studies should reveal the morphology of physiologically characterized SHT axons and their collaterals bilaterally in the diencephalon.